Fluoroelastomers, particularly the copolymers of vinylidene fluoride with other ethylenically unsaturated halogenated monomers, such as hexafluoropropene, have particular utility in high temperature applications, such as seals, gaskets, and linings—see, for example, Brullo, R. A., “Fluoroelastomer Rubber for Automotive Applications,” Automotive Elastomer & Design, June 1985, “Fluoroelastomer Seal Up Automotive Future,” Materials Engineering, October 1988, and “Fluorocarbon Elastomers,” Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 8, pp. 990–1005 (4th. ed., John Wiley & Sons, 1993).
Fluoroelastomers when cured have good resistance to damage by heat, solvents, corrosive chemicals, and steam. However, in the manufacture of molded products from these polymers, particularly in modern manufacturing methods, e.g., injection molding, the cured polymers generally adhere to the surface of the mold even when mold release agents are sprayed on the mold cavity or incorporated in the polymer, and the shaped article is frequently torn or damaged when removed from the mold. Also, the incorporation of a mold release agent into the polymer can have serious adverse effects on the physical properties of the cured composition, for example, Mooney Scorch and compression set, which can limit the successful commercial use of the cured composition. Deposits of polymer on the mold cavity surface (“mold fouling”) and poor release of the shaped vulcanizate from the mold are major reasons for defects, resulting in rejection of the shaped article which adds to the expense of manufacture of such molded articles
Furthermore, the processability in terms of ease of mixing, moulding cycle times and throughput in case of extrusion is fairly poor compared to conventional elastomers because of the poor flow characteristics of the fluoroelastomer during processing. Processing aids have been suggested to improve the processability of fluoroelastomers such as for example disclosed in EP 691 371.
However, although such processing aids are effective, there continues to exist a need to find further suitable processing aids. However, such processing aids should not adversely affect the properties of the fluoroelastomer article produced and should desirably be compatible with existing manufacturing procedures, in particular, they preferably should not contaminate the equipment or contribute to flow defects, such as knit-lines. Furthermore, the processing aids should desirably not affect the scorching safety or preferably improve the scorching safety.
Knit-lines are molding defects resulting from the incomplete joining of two or more polymer streams during the molding process. While changing the mold temperature or compound temperature may influence this behavior, incompatible process aids may have a larger effect. To the extent any process aid or additive exudes to the surface of the molten polymer stream during processing, such a layer of exuded process aid or additive may serve to prevent the proper joining or knitting of the two molten polymer streams as they meet in a mold cavity. This incomplete or defective joining of the polymer streams results in a defect in the finished part called a knit-line and is a likely point of failure in use.
Many conventional fluoroelastomer compositions tend toward “scorching” behavior, i.e., the premature crosslinking or partial cure of the composition when exposed to elevated temperatures or conditions of high shear. This scorching behavior particularly is pronounced when the fluoroelastomer is injection molded, wherein scorching is characterized by a premature cure initiation occurring prior to and during injection of the compounded composition into a mold. The point of cure initiation for injection-molded fluoroelastomers may be defined as the time after which the compounded fluoroelastomer is subjected to injection molding conditions (i.e., upon introduction into an injection barrel at a temperature above approximately 70–90° C. and/or while injecting the compound into the mold under high shear at temperatures between about 180 and 200° C.) when the curing compound begins to gel or harden. Such a change in physical properties, particularly the corresponding viscosity increase, can greatly reduce processing efficiency by hindering the ability to inject the compounded mixture into a mold. Scorching phenomena also produce high levels of waste product; because a cured fluoroelastomer is very difficult to reprocess, any fluoroelastomer that cures outside the mold cavity must usually be discarded.